OES 



OES 



it then makes a stop, taking tim- to 

 spread out quite flat ; it may be half an 

 hour from the first bursting- of the calyx at 

 I Bottom to the final expansion of the corol- 

 la, which commonly becomes flaccid in 

 the course of the next day, according to 

 the heat or coolness of the weather; the 

 uppermost flowers appear first in June ; 

 \he stalks keep continually advancing in 

 height, and there is a constant succession 

 of flowers till late in autumn. It is a 

 native of North America. 



OESOPHAGUS, the gula, or gullet, is 

 a membranaceous canal, reaching from 

 the fauces to the stomach, and conveying 

 into it the food taken in at the mouth. 

 Its figure is somewhat like that of a fun- 

 nel, and its upper part is called by 

 anatomists the pharynx. See ANATO- 

 MY. 



OESTRUS, in natural history, gad-fly, 

 a genus of insects of the order Diptera. 

 Mouth with a simple aperture, and not 

 exserted : feelers two, of two articula- 

 tions orbicular at the tip, and seated each 

 side in a depression of the mouth : anten- 

 nae of three articulations, the last subglo- 

 bular, and furnished with a bristle on the 

 fore-part, placed in two hollows on the 

 front. The face of this singular genus is 

 broad, depressed, vesicular, and glaucous, 

 and has some sort of resemblance to the 

 ape kind. They are extremely trouble- 

 some to horses, sheep, and cattle, deposi- 

 ting their eggs in" different parts of the 

 body, and producing very painful tu- 

 mours, and sometimes death. The larvae 

 are without feet, short, thick, and annu- 

 late, and often-furnished with small hooks. 

 There are twelve species, named from 

 the animals which they infest : thus we 

 have O. bovis, O. equi, p. ovis, O. homi- 

 nis, &c. The principal European species 

 is the O. bovis, or ox gad fly, which is the 

 size of a common bee, and is of a pale yel- 

 lowish colour, with the thorax marked 

 with four longitudinal dusky streaks, and 

 the abdomen by a black bar across the 

 middle ; the lip" is covered with tawny 

 orange-coloured hairs ; the wings are 

 pale-brown, and unspotted. The female 

 of this species, when ready to deposit her 

 eggs, fastens on the back of a heifer, or 

 cow, and piercing the skin with the tube 

 situated at the tip of the abdomen, de- 

 posits an egg in the puncture, and then 

 proceeds to another spot at some distance 

 from the former, repeating the same ope- 

 ration, at intervals, on many parts of the 

 animal's back. The pain which this ope- 

 ration occasions is extreme ; and hence 



cattle, as if foreseeing their cruel enemy, 

 are observed to be seized with the mosi 

 violent horror when apprehensive of the 

 approaches of the female oestrus, flying 

 instantly to the nearest pond or pool of 

 water; it having been observed that this 

 insect rarely attacks cattle when standing 

 in water. The eggs are laid in August 

 or September, and the larvae remain till 

 the following summer before they under- 

 go the change to the pupa state. At this 

 period they force themselves out of their 

 respective cells, and falling to the ground, 

 creep beneath the first convenient shel- 

 ter, and lying in an inert state become 

 contracted into an oval form, but without 

 casting the larva skin, which dries and 

 hardens round them. When the included 

 insect is ready for exclusion, it forces 

 open the top of the pupa coat, and emer- 

 ges in its perfect form, having remained 

 within the chrysalis somewhat more than 

 a month. 



We shall give an account of the O. equi, 

 from the Transactions of the Linnsean So- 

 ciety, drawn up with great accuracy by 

 Mr. Clarke. When the female has 

 been impregnated, and the eggs are suffi- 

 ciently mature, she seeks among the hor- 

 ses a subject for her purpose ; and ap- 

 proaching it on the wing, she holds her 

 body nearly upright in the air, and her 

 tail, which is lengthened for the purpose, 

 curved inwards and upwards : in this way 

 she approaches the part where she de- 

 signs to deposit her egg ; and, suspend- 

 ing herself for a few seconds before it, 

 suddenly darts upon it, and leaves her 

 egg adhering to the hair : she hardly ap- 

 pears to settle, but merely touches the 

 hair with the cggheld out on the project- 

 ed point of the abdomen. The egg is 

 made to adhere by means of a glutinous 

 liquid secreted with it. She then leaves 

 the horse at a small distance, and prepares 

 a second egg, and, poising herself before 

 file part, deposits it in the same way. 

 The liquor dries, and the egg becomes 

 firmly glued to the hair : this is repeated 

 by various flies, till four or five hundred 

 eggs are sometimes placed on one horse. 

 The horses, when they become used to 

 this fly, and find that it does them no in- 

 jury, as the Tabani and Conopes, by suck- 

 ing their blood, hardly regard it, and do 

 not appear at all aware of its insidious ob- 

 ject. The skin of the horse is always 

 thrown into a tremulous motion on the 

 touch of this insect, which merely arises 

 from the very great irritability of the 

 skin and cutaneous muscles at this season 



